Two-dimensional grating coupler and methods of making same

ABSTRACT

Disclosed are apparatus and methods for optical coupling. In one example, a described apparatus includes: a planar layer; a grating region comprising an array of scattering elements arranged in the planar layer to form a two-dimensional grating; a first taper structure formed in the planar layer connecting a first side of the grating region to a first waveguide, wherein a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer; and a second taper structure formed in the planar layer connecting a second side of the grating region to a second waveguide, wherein a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer, wherein the first side and the second side are substantially perpendicular to each other.

BACKGROUND

Optical gratings are frequently used to couple light between a waveguideand an optical fiber. Due to extremely different dimensions of thewaveguide and the optical fiber, direct coupling would incur tremendouslight loss. An incoming light to a waveguide is usually in an unknownand arbitrary polarization state, such that a two-dimensional (2D)grating coupler is needed to provide polarization light in eithertransverse magnetic (TM) or transverse magnetic (TE) polarization modefrom the optical fiber to the waveguide.

A conventional 2D grating coupler includes two symmetric taperstructures coupled to a 2D grating. To reduce power loss and to improvecoupling efficiency of the conventional 2D grating coupler, complicateddesigns of the 2D grating have been proposed to find proper output fieldto match a given taper design of the 2D grating coupler, which takeslots of hardware resources and simulation time.

As such, there exists a need to develop a method and apparatus forefficient optical coupling using novel taper designs.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of illustration.

FIG. 1 illustrates an exemplary block diagram of a device, in accordancewith some embodiments of present disclosure.

FIG. 2A illustrates a top view of an exemplary two-dimensional (2D)grating coupler, in accordance with some embodiments of the presentdisclosure.

FIG. 2B illustrates an exemplary grating region comprising an array ofscattering elements in a 2D grating coupler, in accordance with someembodiments of the present disclosure.

FIG. 2C illustrates an exemplary optical fiber coupled to a 2D gratingcoupler with an incident angle, in accordance with some embodiments ofthe present disclosure.

FIG. 3A illustrates an exemplary diagram of a 2D grating coupler withdesigned taper shape and dimensions, in accordance with some embodimentsof the present disclosure.

FIG. 3B illustrates an exemplary diagram of another 2D grating couplerwith designed taper shape and dimensions, in accordance with someembodiments of the present disclosure.

FIG. 4A illustrates a perspective view of a 2D grating coupler, inaccordance with some embodiments of the present disclosure.

FIG. 4B illustrates a cross-sectional view of a 2D grating coupler, inaccordance with some embodiments of the present disclosure.

FIGS. 5A-5K illustrate cross-sectional views of an exemplary gratingcoupler at various stages of a fabrication process, in accordance withsome embodiments of the present disclosure.

FIG. 6 illustrates a flow chart of an exemplary method for making anexemplary 2D grating coupler, in accordance with some embodiments of thepresent disclosure.

FIG. 7A illustrates an exemplary light power performance of a 2D gratingcoupler with different input light wavelengths, in accordance with someembodiments of the present disclosure.

FIG. 7B illustrates an expanded view of an exemplary selected wavelengthrange for input light of a 2D grating coupler, in accordance with someembodiments of the present disclosure.

FIG. 8 illustrates a flow chart of an exemplary method for designing anexemplary 2D grating coupler, in accordance with some embodiments of thepresent disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, it will be understood that when anelement is referred to as being “connected to” or “coupled to” anotherelement, it may be directly connected to or coupled to the otherelement, or one or more intervening elements may be present.

A waveguide surrounded by a cladding layer may confine light based onrefractive index contrast between the materials in the waveguide and thecladding layer. For example, a silicon waveguide with sub-microndimension can confine infrared light (with a wavelength larger thanabout 700 nanometers or 700 nm) due to its strong refractive indexcontrast to its silicon oxide cladding layer, wherein the refractiveindices for silicon and silicon oxide are about 3.47 and 1.45,respectively. To receive or transmit light signals, light needs to becoupled between a waveguide and an optical fiber. While an outgoinglight from a silicon waveguide is usually in transverse magnetic (TE)mode and can be vertically coupled to a fiber using single polarizationgrating coupler, an incoming light to a silicon waveguide is usually inan unknown and arbitrary polarization state, such that a polarizationsplitting grating coupler (PSGC) is needed to provide polarization lightin either transverse magnetic (TM) or transverse magnetic (TE)polarization mode from the optical fiber to the waveguide. In oneembodiment, a PSGC may be a two-dimensional (2D) grating coupler formedby two single polarization grating couplers nearly perpendicular to eachother. Each single polarization grating coupler has a taper structurecoupled to a common 2D grating region, which includes grating lines withscattering elements at the intersection of grating lines. Differentdesigns of the taper structures are disclosed to reduce power loss andimprove light coupling efficiency from the optical fiber to the 2Dgrating coupler.

In one embodiment, beam propagation is simulated as input light to the2D grating coupler based on a plane wave expansion method, to determinea wavelength range for the input light based on the simulated beampropagation to minimize light power loss due to light transmission andreflection with respect to the 2D grating. At the wavelength range,shape and dimensions of the taper structures of the 2D grating couplerare adjusted to fit output light from the 2D grating. For example,gradually increasing values for the length and shift of each taper aresimulated to determine an optimal taper design to minimize light powerloss.

In one embodiment, shapes of the two tapers of the 2D grating couplerare two congruent triangles that are symmetric to each other about adiagonal line crossing the grating region. But the triangles are notisosceles triangles and have a shift from a vertex to a perpendicularbisector of the base side opposite the vertex. The length and shift ofeach triangle may be determined based on a position of an optical fibercoupled to the 2D grating and/or based on an incident angle of anincident light from the optical fiber.

FIG. 1 illustrates an exemplary block diagram of a device 100, inaccordance with some embodiments of present disclosure. It is noted thatthe device 100 is merely an example, and is not intended to limit thepresent disclosure. Accordingly, it is understood that additionalfunctional blocks may be provided in or coupled to the device 100 ofFIG. 1, and that some other functional blocks may only be brieflydescribed herein.

Referring to FIG. 1, the device 100 comprises an electronic die 102, alight source die 104, a photonic die 106, an interposer 110 and aprinted circuit board (PCB) substrate 114. The electronic die 102, lightsource die 104 and the photonic die 106 are coupled together throughinput/output interfaces (not shown) on the interposer 110. In someembodiments, the interposer 110 is fabricated using silicon. In someembodiments, the interposer 110 comprises at least one of the following:interconnecting routing, through silicon via (TSV), and contact pads. Insome embodiments, the interposer 110 is to integrate all componentsincluding the electronic die 102, the light source die 104, and thephotonic die 106 together. In certain embodiments, each of the dies102/104/106 are coupled to the interposer 110 using a flip-chip (C4)interconnection method. In some embodiments, high density soldermicrobumps are used to couple the dies 102/104/106 to the interposer110. Further, the interposer 110 is coupled to the PCB substrate 114through wire bonding 112 or through silicon-vias (TSV) 116 usingsoldering balls. The TSVs 116 can comprise electrically conductive pathsthat extend vertically through the interposer 110 and provide electricalconnectivity between the electronic die 102 and the PCB 114. In someembodiments, the PCB substrate 114 can comprises a support structure forthe device 100, and can comprise both insulating and conductive materialfor isolation devices as well as providing electrical contact for activedevices on the photonic die 106 as well as circuits/devices on theelectronic die 102 via the interposer 110. Further, the PCB substrate114 can provide a thermally conductive path to carry away heat generatedby devices and circuits in the electronic die 102 and the light sourcedie 104.

In some embodiments, the electronic die 102 comprises circuits (notshown) including amplifiers, control circuit, digital processingcircuit, etc., as well as driver circuits for controlling the lightsource 104 or elements in the photonic die 106. In some embodiments, thelight source die 104 comprises a plurality of components (not shown),such as at least one light emitting elements (e.g., a laser or alight-emitting diode), transmission elements, modulation elements,signal processing elements, switching circuits, amplifier, input/outputcoupler, and light sensing/detection circuits. In some embodiments, thelight source die 104 is on the photonic die 106. In some embodiments,the photonic die 106 comprises an optical fiber array 108 attachedthereon, an optical interface and a plurality of fiber-to-chip gratingcouplers 118. In some embodiments, the plurality of fiber-to-chipgrating coupler 118 is configured to couple the light source 106 and theoptical fiber array 108. In some embodiments, the optical fiber array108 comprises a plurality of optical fibers and each of them can be asingle-mode or a multi-mode optical fiber. In some embodiments, theoptical fiber array 108 can be epoxied on the photonic die 106.

In some embodiments, each of the plurality of fiber-to-chip gradingcoupler 118 enables the coupling of optical signals between the opticalfiber array 108 and the light source die 102 or correspondingphotodetectors on the photonic die 106. Each of the plurality offiber-to-chip grating couplers 118 comprises a plurality of gratings anda waveguide with designs to improve coupling efficiency between theoptical fiber on the corresponding waveguide, which are discussed indetails below in various embodiments of the present disclosure.

During operation, optical signals received from a remote server attachedon one end of the optical fiber array 108 can be coupled through thefiber-to-chip grating couplers 118 attached to the other end of theoptical fiber array 108 to the corresponding photodetectors on thephotonic die 106. Alternatively, optical signals received from the lightsource die 104 can be coupled through the fiber-to-chip grating couplers118 to the optical fiber array 108 which can be further transmitted tothe remote server. In one embodiment, the fiber-to-chip grating coupler118 may be a two-dimensional (2D) grating coupler.

FIG. 2A illustrates a top view of an exemplary 2D grating coupler 200,in accordance with some embodiments of the present disclosure. As shownin FIG. 2A, the 2D grating coupler 200 is formed by two singlepolarization grating couplers nearly perpendicular to each other. Eachsingle polarization grating coupler has a respective taper structure anda shared grating region 230. The first single polarization gratingcoupler includes a first taper structure 210 and the shared gratingregion 230; and the second single polarization grating coupler includesa second taper structure 220 and the shared grating region 230. Thegrating region 230 comprises an array of scattering elements 232arranged in the planar layer to form a 2D grating.

In one embodiment, the first taper structure 210, the second taperstructure 220 and the shared grating region 230 are all formed in aplanar layer, which may be a semiconductor layer, e.g. a silicon layeron a silicon-on-insulator (SOI) substrate. In one embodiment, the firsttaper structure 210 is formed in the planar layer connecting a firstside 212 of the 2D grating 230 to a first waveguide 218 in the planarlayer; and the second taper structure 220 is formed in the planar layerconnecting a second side 222 of the 2D grating 230 to a second waveguide228 in the planar layer. The first side 212 and the second side 222 aresubstantially perpendicular to each other.

FIG. 2B illustrates an expanded view of the exemplary grating region 230comprising an array of scattering elements 232 in a 2D grating coupler,in accordance with some embodiments of the present disclosure. As shownin FIG. 2B, a shape of the grating region 230 may be a square in theplanar layer. In one embodiment, the array of scattering elements arearranged in the planar layer at a plurality of intersections of a firstset of straight lines 216 crossing with a second set of straight lines226. Each of the first set of straight lines 216 is parallel to thefirst side 212 of the grating region 230; and each of the second set ofstraight lines 226 is parallel to the second side 222 of the gratingregion 230. Any numbers of straight lines 216, 226 and any numbers ofscattering elements 232 on each straight line can be used and are withinthe scope of the present disclosure. As shown in FIG. 2A and FIG. 2B,each scattering element 232 in the array of scattering elements has asame circular shape with a same size in the planar layer. In anotherembodiment, each scattering element in the array of scattering elementshas a same square shape with a same size in the planar layer. As shownin FIG. 2A and FIG. 2B, the array of scattering elements 232 are evenlydistributed in the planar layer such that there is a same distancebetween centers of every two adjacent scattering elements 232 alongeither a first direction (the X direction) perpendicular to the firstside 212 of the grating region 230 or a second direction (the Ydirection) perpendicular to the second side 222 of the grating region230.

Referring back to FIG. 2A, the 2D grating coupler 200 may scatterincident light received from the first waveguide 218 in a directionperpendicular to the first side 212 along the —X direction; and may alsoscatter incident light received from the second waveguide 228 in adirection perpendicular to the second side 222 along the —Y direction.

In one embodiment, the 2D grating coupler 200 scatters incident lightreceived from a fiber having a fiber mode 250 attached to the 2Dgrating. In one embodiment, the 2D grating 230 of the coupler 200 isconfigured for receiving an incident light from an optical fiber 290with an incident angle 292, as shown in FIG. 2C. The incident angle 292is measured in plane of incidence between an axis of the optical fiber290 and the Z direction, a direction perpendicular to the planar layer.The plane of incidence is a plane which contains the surface normal ofthe planar layer and the propagation vector of the incident light. Thatis, the plane of incidence is the plane formed by the Z direction andthe X direction. In one embodiment, the incident angle 292 is non-zero.Both the optical fiber 290 and the 2D grating coupler 200 may beattached to or included in a photonic die on a substrate. The 2D gratingcoupler 200 includes an array of scattering elements 232 on the photonicdie for transmitting light between the photonic die and the opticalfiber 290.

The 2D grating coupler 200 may be configured for splitting the incidentlight received from the fiber on top of the planar layer to a parallelpolarization component and an orthogonal polarization component. In oneembodiment, the 2D grating coupler 200 couples the parallel polarizationcomponent to the first waveguide 218 via the first taper structure 210;and couples the orthogonal polarization component to the secondwaveguide 228 via the second taper structure 220. Alternatively, the 2Dgrating coupler 200 can couple the orthogonal polarization component tothe first waveguide 218 via the first taper structure 210; and couplesthe parallel polarization component to the second waveguide 228 via thesecond taper structure 220.

As shown in FIG. 2A, the first taper structure 210 has a reducing firstwidth from the first side 212 to the first waveguide 218; and the secondtaper structure 220 has a reducing second width from the second side 222to the second waveguide 228. In one embodiment, the first taperstructure 210 is configured for transmitting a first portion of theincident light from the fiber to the first waveguide 218 to achieve aminimum insertion loss; and the second taper structure 220 is configuredfor transmitting a second portion of the incident light to the secondwaveguide 228 to achieve a minimum insertion loss. The first portion ofthe incident light is substantially a parallel polarization component ofthe incident light, and the second portion of the incident light issubstantially an orthogonal polarization component of the incidentlight. Each of the parallel polarization component and the orthogonalpolarization component comprises a polarized light split from theincident light. The polarized light has either a transverse magnetic(TM) polarization mode or a transverse magnetic (TE) polarization mode.

FIG. 3A illustrates an exemplary block diagram of a 2D grating coupler300-1, which may be implemented as the 2D grating coupler 200 in FIG.2A, with designed taper shape and dimensions, in accordance with someembodiments of the present disclosure. As shown in FIG. 3A, the 2Dgrating coupler 300-1 includes: a grating region 230 in a planar layer,a first taper structure 310-1 in the planar layer connecting a firstside 212 of the grating region 230 to a first waveguide, a second taperstructure 320-1 in the planar layer connecting a second side 222 of thegrating region 230 to a second waveguide. In one embodiment, the gratingregion 230 has a square shape; and the first side 212 and the secondside 222 are substantially perpendicular to each other.

In the example shown in FIG. 3A, a shape of the first taper structure310-1 is a first triangle that is asymmetric about any lineperpendicular to the first side 212 of the grating region 230 in theplanar layer; and a shape of the second taper structure 320-1 is asecond triangle that is asymmetric about any line perpendicular to thesecond side 222 of the grating region 230 in the planar layer. In oneembodiment, the first triangle and the second triangle are congruent.Although each of the first taper structure 310-1 and the second taperstructure 320-1 may have a shape of a trapezoid in practice, a design ofthe triangle shape automatically determines a corresponding design ofthe trapezoid, with given widths of the first and second waveguides. Forexample, once the shape and dimensions of the first triangle 310-1 aredetermined, one can determine a corresponding trapezoid having a topside 318-1 with a given width w1 and having three other sides residingon the three sides of the first triangle 310-1; once the shape anddimensions of the second triangle 320-1 are determined, one candetermine a corresponding trapezoid having a top side 328-1 with a givenwidth w2 and having three other sides residing on the three sides of thesecond triangle 320-1. The width w1 may be determined based on a widthof the first waveguide; and the width w2 may be determined based on awidth of the second waveguide. As such, the rest of the application willfocus on the design of the triangles, instead of trapezoids.

As shown in FIG. 3A, the first triangle has a first base side residingon the first side 212 of the grating region 230, has a first vertex313-1 opposite to the first base side 212, and has a first length L1316-1 along the X direction; and the second triangle has a second baseside residing on the second side 222 of the grating region 230, has asecond vertex 323-1 opposite to the second base side 222, and has asecond length L2 326-1 along the Y direction. As shown in FIG. 3A, thefirst vertex 313-1 has a first distance or shift S1 315-1 to aperpendicular bisector 312-1 of the first base side 212 in the planarlayer; and the second vertex 323-1 has a second distance or shift S2325-1 to a perpendicular bisector 322-1 of the second base side 222 inthe planar layer.

In one embodiment, the first triangle and the second triangle aresymmetric to each other about a diagonal line 235 crossing the gratingregion 230. In this case, the first length L1 316-1 and the secondlength L2 326-1 are equal to each other; the first distance S1 315-1 andthe second distance S2 325-1 are equal to each other.

According to various embodiments, the values of the first length L1316-1, the second length L2 326-1, the first distance S1 315-1 and thesecond distance S2 325-1 can be determined based on the incident angleand a position of the optical fiber relative to the 2D grating 230.According to various embodiments, each of the first length L1 316-1 andthe second length L2 326-1 is between 20 and 500 micrometers; and eachof the first distance S1 315-1 and the second distance S2 325-1 isbetween 0 and 20 micrometers. In some embodiments, each of the firstlength L1 316-1 and the second length L2 326-1 is between 100 and 150micrometers; and each of the first distance S1 315-1 and the seconddistance S2 325-1 is between 0 and 10 micrometers. It can be understoodthat once the first length L1 316-1, the second length L2 326-1, thefirst distance S1 315-1 and the second distance S2 325-1 are determined,the top angle a of the top vertex 313-1 and the top angle b of the topvertex 323-1 are automatically determined as well.

FIG. 3B illustrates an exemplary diagram of another 2D grating coupler300-2, which may be implemented as the 2D grating coupler 200 in FIG.2A, with designed taper shape and dimensions, in accordance with someembodiments of the present disclosure. As shown in FIG. 3B, the 2Dgrating coupler 300-2 includes: a grating region 230 in a planar layer,a first taper structure 310-2 in the planar layer connecting a firstside 212 of the grating region 230 to a first waveguide, a second taperstructure 320-2 in the planar layer connecting a second side 222 of thegrating region 230 to a second waveguide. In one embodiment, the gratingregion 230 has a square shape; and the first side 212 and the secondside 222 are substantially perpendicular to each other.

In the example shown in FIG. 3B, a shape of the first taper structure310-2 is a first triangle that is asymmetric about any lineperpendicular to the first side 212 of the grating region 230 in theplanar layer; and a shape of the second taper structure 320-2 is asecond triangle that is asymmetric about any line perpendicular to thesecond side 222 of the grating region 230 in the planar layer. In oneembodiment, the first triangle and the second triangle are congruent.Although each of the first taper structure 310-2 and the second taperstructure 320-2 may have a shape of a trapezoid in practice, a design ofthe triangle shape automatically determines a corresponding design ofthe trapezoid, with given widths of the first and second waveguides. Forexample, once the shape and dimensions of the first triangle 310-2 aredetermined, one can determine a corresponding trapezoid having a topside 318-2 with a given width w3 and having three other sides residingon the three sides of the first triangle 310-2; once the shape anddimensions of the second triangle 320-2 are determined, one candetermine a corresponding trapezoid having a top side 328-2 with a givenwidth w4 and having three other sides residing on the three sides of thesecond triangle 320-2. The width w3 may be determined based on a widthof the first waveguide; and the width w4 may be determined based on awidth of the second waveguide. As such, the rest of the application willfocus on the design of the triangles, instead of trapezoids.

As shown in FIG. 3B, the first triangle has a first base side residingon the first side 212 of the grating region 230, has a first vertex313-2 opposite to the first base side 212, and has a first length L3316-2 along the X direction; and the second triangle has a second baseside residing on the second side 222 of the grating region 230, has asecond vertex 323-2 opposite to the second base side 222, and has asecond length L4 326-2 along the Y direction. As shown in FIG. 3B, thefirst vertex 313-2 has a first distance or shift S3 315-2 to aperpendicular bisector 312-2 of the first base side 212 in the planarlayer; and the second vertex 323-2 has a second distance or shift S4325-2 to a perpendicular bisector 322-2 of the second base side 222 inthe planar layer.

In one embodiment, the first triangle and the second triangle aresymmetric to each other about a diagonal line 235 crossing the gratingregion 230. In this case, the first length L3 316-2 and the secondlength L4 326-2 are equal to each other; the first distance S3 315-2 andthe second distance S4 325-2 are equal to each other.

According to various embodiments, the values of the first length L3316-2, the second length L4 326-2, the first distance S3 315-2 and thesecond distance S4 325-2 can be determined based on the incident angleand a position of the optical fiber relative to the 2D grating 230.According to various embodiments, each of the first length L3 316-2 andthe second length L4 326-2 is between 20 and 500 micrometers; and eachof the first distance S3 315-2 and the second distance S4 325-2 isbetween 0 and 20 micrometers.

While the vertex 313-1 in FIG. 3A has a shift S1 above the perpendicularbisector 312-1 along the —Y direction, the vertex 313-2 in FIG. 3B has ashift S3 below the perpendicular bisector 312-2 along the Y direction.While the vertex 323-1 in FIG. 3A has a shift S2 to the left of theperpendicular bisector 322-1 along the —X direction, the vertex 323-2 inFIG. 3B has a shift S4 to the right of the perpendicular bisector 322-2along the X direction. The shift of the top vertex of each tapertriangle can be designed based on a position of the optical fibercoupled to the 2D grating and/or an incident angle of incident lightfrom the optical fiber.

FIG. 4A illustrates a perspective view of a 2D grating coupler 400, inaccordance with some embodiments of the present disclosure. As shown inFIG. 4A, the 2D grating coupler 400 includes an array of scatteringelements and two tapers formed in a semiconductor layer 430. In oneembodiment, each scattering elements 431, 432, 433 comprise a dielectricmaterial such as silicon oxide, while the semiconductor layer 430comprises a semiconductor material such as silicon. In the illustratedembodiment, the semiconductor layer 430 is fabricated on an insulationlayer 420 which is formed on a semiconductor substrate 410.

FIG. 4B illustrates a cross-sectional view of a 2D grating coupler 400along the direction A-A′ in FIG. 4A, in accordance with some embodimentsof the present disclosure. In the illustrated embodiments, the 2Dgrating coupler 400 fabricated on a semiconductor substrate 410comprises a multi-layered structure comprising an insulation layer 420and a semiconductor layer 430. In the illustrated embodiment, thesemiconductor substrate 410 comprises silicon. The insulation layer 420comprises a dielectric material such as silicon oxide, and is fabricatedon the semiconductor substrate 410 using chemical vapor deposition,physical vapor deposition, etc. In some embodiments, the insulationlayer 420 can be replaced by other types of dielectric materials, suchas Si3N4, SiO2 (e.g., quartz, and glass), Al2O3, and H2O, according tovarious embodiments of the present disclosure. In some embodiments, thesemiconductor layer 430 comprises silicon and is deposited on theinsulation layer 420 using chemical vapor deposition. In someembodiments, the semiconductor substrate 410, the insulation layer 420and the semiconductor layer 430 are formed as a silicon-on-insulator(SOI) substrate.

In some embodiments, the scattering elements 431, 432, 433 are formedaccording to a predetermined pattern as shown in FIG. 4A and FIG. 2B. Insome embodiments, the scattering elements 431, 432, 433 are formed aspart of a cladding layer comprising silicon oxide. In some embodiments,the cladding layer can comprise other types of dielectric materialsaccording to different applications, including polycrystalline siliconand silicon nitride.

In some embodiments, the 2D grating coupler 400 may further comprise: abottom reflection layer that is located between the semiconductorsubstrate 410 and the insulation layer 420 and comprises at least one ofthe following: Al, Cu, Ni, and a combination; and/or a top reflectionlayer that is located on the cladding layer and comprises at least oneof the following: Al, Cu, Ni and a combination. In some embodiments, thetop reflection layer only covers the taper structures 402 of the 2Dgrating coupler 400. In some embodiments, the taper structures 402 ofthe 2D grating coupler 400 comprise the same material used in thegrating region 401 of the semiconductor layer 430. In other embodiments,the taper structures 402 comprise a second material that is differentfrom the material used in the grating region 401 of the semiconductorlayer 430.

FIGS. 5A-5K illustrate cross-sectional views of an exemplary gratingcoupler 500 at various stages of a fabrication process, in accordancewith some embodiments of the present disclosure. FIG. 5A is across-sectional view of the grating coupler 500-1 including a firstlayer 510 and a second layer 520 disposed on the first layer 510, at oneof the various stages of fabrication, according to some embodiments ofthe present disclosure. The first layer 510 may be formed of silicon oranother semiconductor material as a substrate. The second layer 520 maybe formed of silicon oxide or another oxide material as an insulationlayer.

FIG. 5B is a cross-sectional view of the grating coupler 500-2 includinga semiconductor layer 530 formed on the insulation layer 520 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The semiconductor layer 530 may be formed by anepitaxial growth of a semiconductor material, e.g. silicon, on theinsulation layer 520.

FIG. 5C is a cross-sectional view of the grating coupler 500-3 includinga hard mask layer 540 deposited on the semiconductor layer 530 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The hard mask layer 540 on the semiconductor layer530 may comprise an organic or inorganic material.

FIG. 5D is a cross-sectional view of the grating coupler 500-4 includinga photoresist layer 550 deposited on the hard mask layer 540 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The photoresist layer 550 on the hard mask layer 540may comprise a photoresist material.

FIG. 5E is a cross-sectional view of the grating coupler 500-5 includingpatterned portions of the photoresist layer 550, formed on the hard masklayer 540 at one of the various stages of fabrication, according to someembodiments of the present disclosure. The photoresist layer 550 ispatterned according a predetermined pattern, e.g. by removing portionscorresponding to the scattering elements shown in FIGS. 1-4, based onwaveguide lithography and development. Based on the pattern, the gratingcoupler can be divided into portions including the grating region 501and the taper structures 502.

FIG. 5F is a cross-sectional view of the grating coupler 500-6 includingpatterned portions of the hard mask layer 540, formed at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. Because the photoresist layer 550 was patterned tohave openings over the hard mask layer 540, the portions of the hardmask layer 540 that are exposed by the photoresist layer 550 areremoved, e.g., via a wet or dry etch procedure. For simplicity ofillustration, three openings are shown in the grating region 501. It canbe understood that any numbers of openings in the grating region 501 canbe fabricated according a predetermined pattern and are within the scopeof the present disclosure.

FIG. 5G is a cross-sectional view of the grating coupler 500-7, wherethe photoresist layer 550 is removed at one of the various stages offabrication, according to some embodiments of the present disclosure.For example, the photoresist layer 550 may be removed by a resiststripping.

FIG. 5H is a cross-sectional view of the grating coupler 500-8 includingan array of etched regions 532, 534, 536, formed at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. Because the hard mask layer 540 was patterned to haveopenings over the semiconductor layer 530, the portions of thesemiconductor layer 530 that are exposed by the hard mask layer 540 areremoved, e.g., via a wet or dry etch procedure, to form the array ofetched regions 532, 534, 536.

In some embodiments, surfaces of the etched regions 532, 534, 536 may besmoothed by: oxidizing the silicon surfaces of the etched regions 532,534, 536; etching the silicon oxide surfaces; and repeating theoxidizing and the etching several times to smooth the surfaces of theetched regions 532, 534, 536.

FIG. 5I is a cross-sectional view of the grating coupler 500-9, wherethe hard mask layer 540 is removed at one of the various stages offabrication, according to some embodiments of the present disclosure.For example, the hard mask layer 540 may be removed by a resiststripping.

FIG. 5J is a cross-sectional view of the grating coupler 500-10including a cladding layer 560, which is formed at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. The cladding layer 560 may be formed by depositing adielectric material such as silicon oxide over the semiconductor layer530 and into the array of etched regions 532, 534, 536.

FIG. 5K is a cross-sectional view of the grating coupler 500-11, wherethe top portion of the cladding layer 560 is polished at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. The top portion of the cladding layer 560 may bepolished to form an array of scattering elements 562, 564, 566 in thearray of etched regions 532, 534, 536, e.g. based on achemical-mechanical polishing process.

FIG. 6 illustrates a flow chart of an exemplary method 600 for making anexemplary 2D grating coupler, e.g. any one of the 2D grating couplersdisclosed in FIGS. 1-4, in accordance with some embodiments of thepresent disclosure. At operation 602, an insulation layer is formed on asemiconductor substrate. At operation 604, a semiconductor material isepitaxially grown on the insulation layer to form a semiconductor layer.At operation 606, a hard mask is deposited on the semiconductor layer.At operation 608, a photoresist is deposited on the hard mask. Atoperation 610, a pattern is determined based on shape and dimensions ofthe grating and tapers. In various embodiments, the tapers may havedifferent shapes and sizes as shown in FIGS. 1-4. The tapers may bedesigned based on simulation in accordance with desired fiber positionand incident angle.

At operation 612, the photoresist is patterned according to the pattern.At operation 614, the hard mask is etched according to the pattern. Atoperation 616, the semiconductor layer is etched to form an array ofetched regions. At operation 618, the surfaces of the etched regions aresmoothed, e.g. by repetitively oxidizing the surfaces and etching theoxidized surfaces. At operation 620, the etched hard mask on thesemiconductor layer is removed. At operation 622, a dielectric materialis deposited into the array of etched regions and over the semiconductorlayer. At operation 624, the top dielectric material is polished to forman array of scattering elements. The order of the operations in FIG. 6may be changed according to various embodiments of the present teaching.

FIG. 7A illustrates an exemplary light power performance of a 2D gratingcoupler, e.g. any one of the 2D grating couplers disclosed in FIGS. 1-4,with different input light wavelengths, in accordance with someembodiments of the present disclosure. This can be achieved bysimulating beam propagation as input light to the 2D grating couplerbased on a plane wave expansion method. As discussed above, the taperdesign aims to minimize light power loss when transmitting light from afiber to the 2D grating coupler. The power loss may be due to a lighttransmission through the 2D grating (e.g. along the −Z direction inFIGS. 2-4); and due to a light reflection back from the 2D grating (e.g.along the Z direction in FIGS. 2-4). While the curves 710, 720 representlight power loss due to light transmission through and light reflectionback from the 2D grating respectively, the curve 730 represents a sum ofthe two power loss. Based on the simulated beam propagation, an optimalwavelength range 701 can be selected for the input light to minimizelight power loss due to light transmission and reflection with respectto the 2D grating.

FIG. 7B illustrates an expanded view of the selected wavelength range701 for input light of the 2D grating coupler, in accordance with someembodiments of the present disclosure. As shown in FIG. 7B, the minimalvalue point 702 of the total power loss 730 corresponds to a wavelengthof about 1310 nanometers.

In one embodiment, shape and dimensions of each taper structure of the2D grating coupler can be adjusted to fit output light from the 2Dgrating, based on an incident light with the selected wavelength ofabout 1310 nanometers. For example, various values can be simulated forthe length and shift of each taper to maximize light power received atthe corresponding waveguide from the taper. In one example, graduallyincreasing or decreasing values can be simulated for the length andshift of each taper. According to various embodiments, the graduallyincreasing or decreasing values for the length of tapers may be between20 and 500 micrometers; and the gradually increasing or decreasingvalues for the shift of tapers may be between 0 and 20 micrometers.

In some embodiments, each of the first length L1 316-1 and the secondlength L2 326-1 is between 20 and 500 micrometers; and each of the firstdistance S1 315-1 and the second distance S2 325-1 is between 0 and 20micrometers.

FIG. 8 illustrates a flow chart of an exemplary method 800 for designingan exemplary 2D grating coupler, e.g. any one of the 2D grating couplersdisclosed in FIGS. 1-4, in accordance with some embodiments of thepresent disclosure. At operation 802, beam propagation is simulated asinput light to a 2D grating coupler based on a plane wave expansionmethod. At operation 804, a wavelength range is determined for the inputlight based on the simulated beam propagation to minimize light powerloss due to light transmission and reflection with respect to the 2Dgrating. At operation 806, based on the wavelength range, shape anddimensions of each taper structure of the 2D grating coupler areadjusted to fit output light from the 2D grating, based on simulatedgradually increasing values. The order of the operations in FIG. 8 maybe changed according to various embodiments of the present teaching.

In one embodiment, an apparatus for optical coupling is disclosed. Theapparatus includes: a planar layer; a grating region comprising an arrayof scattering elements arranged in the planar layer to form atwo-dimensional (2D) grating; a first taper structure formed in theplanar layer connecting a first side of the grating region to a firstwaveguide, wherein a shape of the first taper structure is a firsttriangle that is asymmetric about any line perpendicular to the firstside of the grating region in the planar layer; and a second taperstructure formed in the planar layer connecting a second side of thegrating region to a second waveguide, wherein a shape of the secondtaper structure is a second triangle that is asymmetric about any lineperpendicular to the second side of the grating region in the planarlayer, wherein the first side and the second side are substantiallyperpendicular to each other.

In another embodiment, a method for designing a two-dimensional (2D)grating coupler is disclosed. The method includes: simulating beampropagation as input light to the 2D grating coupler based on a planewave expansion method, wherein the 2D grating coupler comprises: aplanar layer, a grating region comprising an array of scatteringelements arranged in the planar layer to form a 2D grating, a firsttaper structure in the planar layer connecting a first side of thegrating region to a first waveguide, and a second taper structure in theplanar layer connecting a second side of the grating region to a secondwaveguide; determining a wavelength range for the input light based onthe simulated beam propagation to minimize light power loss due to lighttransmission and reflection with respect to the 2D grating; andadjusting, based on the wavelength range, shape and dimensions of eachof the first taper structure and the second taper structure, to fitoutput light from the 2D grating.

In yet another embodiment, a method for forming an optical coupler isdisclosed. The method includes: forming an insulation layer on asemiconductor substrate; epitaxially growing a semiconductor material onthe insulation layer to form a semiconductor layer; etching, accordingto a predetermined pattern, the semiconductor layer to form: an array ofetched holes in the semiconductor layer to form a grating region, afirst taper structure extending from a first side of the grating region,wherein a shape of the first taper structure in the semiconductor layeris a first triangle that is asymmetric about any line perpendicular tothe first side of the grating region, and a second taper structureextending from a second side of the grating region, wherein a shape ofthe second taper structure in the semiconductor layer is a secondtriangle that is asymmetric about any line perpendicular to the secondside of the grating region, wherein the first side and the second sideare substantially perpendicular to each other; and depositing adielectric material into the array of etched regions to form an array ofscattering elements in the semiconductor layer, wherein the scatteringelements are arranged to form a two-dimensional (2D) grating.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. An apparatus for optical coupling, comprising: aplanar layer; a grating region comprising an array of scatteringelements arranged in the planar layer to form a two-dimensional (2D)grating; a first taper structure formed in the planar layer connecting afirst side of the grating region to a first waveguide, wherein a shapeof the first taper structure is a first triangle that is asymmetricabout any line perpendicular to the first side of the grating region inthe planar layer; and a second taper structure formed in the planarlayer connecting a second side of the grating region to a secondwaveguide, wherein a shape of the second taper structure is a secondtriangle that is asymmetric about any line perpendicular to the secondside of the grating region in the planar layer, wherein the first sideand the second side are substantially perpendicular to each other. 2.The apparatus of claim 1, wherein a shape of the grating region is asquare in the planar layer.
 3. The apparatus of claim 2, wherein: thearray of scattering elements are arranged in the planar layer at aplurality of intersections of a first set of straight lines crossingwith a second set of straight lines; each of the first set of straightlines is parallel to the first side of the grating region; and each ofthe second set of straight lines is parallel to the second side of thegrating region.
 4. The apparatus of claim 1, wherein each scatteringelement in the array of scattering elements has a same square shape witha same size in the planar layer.
 5. The apparatus of claim 1, whereineach scattering element in the array of scattering elements has a samecircular shape with a same size in the planar layer.
 6. The apparatus ofclaim 1, wherein the first triangle and the second triangle arecongruent.
 7. The apparatus of claim 1, wherein the first triangle andthe second triangle are symmetric to each other about a diagonal linecrossing the grating region.
 8. The apparatus of claim 1, wherein: thearray of scattering elements are evenly distributed in the planar layersuch that there is a same distance between centers of every two adjacentscattering elements along either a first direction perpendicular to thefirst side of the grating region or a second direction perpendicular tothe second side of the grating region.
 9. The apparatus of claim 8,wherein: the first triangle has a first base side residing on the firstside of the grating region, has a first vertex opposite to the firstbase side, and has a first length along the first direction; the firstvertex has a first distance to a perpendicular bisector of the firstbase side in the planar layer; the second triangle has a second baseside residing on the second side of the grating region, has a secondvertex opposite to the second base side, and has a second length alongthe second direction; the second vertex has a second distance to aperpendicular bisector of the second base side in the planar layer; andthe first distance and the second distance are equal to each other. 10.The apparatus of claim 9, wherein: the 2D grating is configured forreceiving an incident light from an optical fiber with an incident anglethat is non-zero; the incident angle is measured in plane of incidencebetween an axis of the optical fiber and a direction perpendicular tothe planar layer; and the first length, the second length, the firstdistance and the second distance are determined based on the incidentangle and a position of the optical fiber relative to the 2D grating.11. The apparatus of claim 10, wherein: the first taper structure isconfigured for transmitting a first portion of the incident light to thefirst waveguide to achieve a minimum insertion loss; and the secondtaper structure is configured for transmitting a second portion of theincident light to the second waveguide to achieve a minimum insertionloss, wherein the first portion of the incident light is substantially aparallel polarization component of the incident light, the secondportion of the incident light is substantially an orthogonalpolarization component of the incident light, each of the parallelpolarization component and the orthogonal polarization componentcomprises a polarized light, and the polarized light has a transversemagnetic (TM) polarization mode or a transverse magnetic (TE)polarization mode split from the incident light.
 12. The apparatus ofclaim 9, wherein: each of the first length and the second length isbetween 20 and 500 micrometers; and each of the first distance and thesecond distance is between 0 and 20 micrometers.
 13. A two-dimensional(2D) grating coupler, comprising: a planar layer; a grating regioncomprising an array of scattering elements arranged in the planar layerto form a 2D grating; a first taper structure in the planar layerconnecting a first side of the grating region to a first waveguide; anda second taper structure in the planar layer connecting a second side ofthe grating region to a second waveguide, wherein each of the firsttaper structure and the second taper structure has a shape anddimensions to minimize light power loss due to light transmission andreflection with respect to the 2D grating.
 14. The 2D grating coupler ofclaim 13, wherein: a shape of the first taper structure is a firsttriangle that is asymmetric about any line perpendicular to the firstside of the grating region in the planar layer; a shape of the secondtaper structure is a second triangle that is asymmetric about any lineperpendicular to the second side of the grating region in the planarlayer; and the first side and the second side are substantiallyperpendicular to each other.
 15. The 2D grating coupler of claim 14,wherein: the first triangle has a first length from a first base sideresiding on the first side to a first vertex opposite the first baseside, and has a first shift from the first vertex to a perpendicularbisector of the first base side; and the second triangle has a secondlength from a second base side residing on the second side to a secondvertex opposite the second base side, and has a second shift from thesecond vertex to a perpendicular bisector of the second base side. 16.The 2D grating coupler of claim 15, wherein: the first length, thesecond length, the first shift and the second shift have values tomaximize light power in the first waveguide and the second waveguide.17. The 2D grating coupler of claim 16, wherein: the values of the firstlength and the second length are between 20 and 500 micrometers; and thevalues of the first shift and the second shift are between 0 and 20micrometers.
 18. A method for forming an optical coupler, comprising:forming an insulation layer on a semiconductor substrate; epitaxiallygrowing a semiconductor material on the insulation layer to form asemiconductor layer; etching, according to a predetermined pattern, thesemiconductor layer to form: an array of etched holes in thesemiconductor layer to form a grating region, a first taper structureextending from a first side of the grating region, wherein a shape ofthe first taper structure in the semiconductor layer is a first trianglethat is asymmetric about any line perpendicular to the first side of thegrating region, and a second taper structure extending from a secondside of the grating region, wherein a shape of the second taperstructure in the semiconductor layer is a second triangle that isasymmetric about any line perpendicular to the second side of thegrating region, wherein the first side and the second side aresubstantially perpendicular to each other; and depositing a dielectricmaterial into the array of etched regions to form an array of scatteringelements in the semiconductor layer, wherein the scattering elements arearranged to form a two-dimensional (2D) grating.
 19. The method of claim18, wherein: a shape of the grating region is a square in thesemiconductor layer; and the first triangle and the second triangle aresymmetric to each other about a diagonal line crossing the gratingregion in the semiconductor layer.
 20. The method of claim 18, wherein:the semiconductor material comprises silicon; and the dielectricmaterial comprises silicon oxide.